2.1. TGF-.beta.
Transforming growth factor-Beta (TGF-.beta.) is a member of a recently described family of polypeptides that regulate cellular differentiation and proliferation. Other members of this family include Mullerian inhibitory substance (Cate et al., 1986, Cell 45:685-698), the inhibins (Mason et al., 1985, Nature 318:659-663) and a protein predicted from a transcript of the decapentaplegic gene complex of Drosophila (Padgett et al., 1987, Nature 325:81-84).
Four types of TGF-.beta. have been identified and designated TGF-1, TGF-.beta.2, TGF-.beta.1.2, and TGF-.beta.3. The first described type, TGF-.beta.1, consists of two identical disulfide linked subunits having molecular weights of 13,000 (Assoian et al., 1983, J. Biol. Chem. 258:7155-7160; Frolik et al, 1983, Proc. Natl. Acad. Sci. USA 80:3676-3680; Frolik et al., 1984, J. Biol. Chem. 260:10995-11000). It has been purified from several tissue sources including placenta (Frolik et al., 1983, Nature 325:81-84), blood platelets (Childs et al., 1982, Proc. Natl. Acad. Sci. USA 79:5312-5316; Assoian et al., 1983, J. Biol. Chem. 258:7155-7160) kidney (Roberts et al., 1983, Biochemistry 22:5692-5698), and demineralized bone (Seyedin et al., 1985, Proc. Natl. Acad. Sci. USA 82:119-123). cDNA clones coding for human (Derynck et al., 1985, Nature 316:701-705), mouse (Derynck et al., 1986, J. Biol. Chem. 261:4377-4379 ) and simian (Sharples et al., 1987, DNA 6:239-244) TGF-.beta.1 have been isolated DNA sequence analysis of these clones indicates that TGF-.beta.1, is synthesized as a large precursor polypeptide, the carboxy terminus of which is cleaved to yield the mature TGF-.beta. monomer. Strong sequence homology has been found throughout the TGF-.beta.1, precursor protein from all of the above sources.
In the presence of 10% serum and epidermal growth factor, TGF-.beta.1, promotes the anchorage independent growth of normal rat kidney fibroblasts (Roberts et al., 1981, Proc. Natl. Acad Sci USA 78:5339-5343; Roberts et al , 1982, Nature 295:417-419; Twardzik et al., 1985, J. Cell. Biochem. 28:289-297); in the presence of 10% serum alone, it is able to induce colony formation of AKR-2B fibroblasts (Tucker et al., 1983, Cancer Res. 43:1518-1586). TGF-.beta.1, has also been shown to cause fetal rat muscle mesenchymal cells to differentiate and produce cartilage specific macromolecules (Seyedin et al., 1986, J. Biol. Chem. 261:5693-5695).
In contrast to its effect on cell proliferation, TGF-.beta.1, purified from human platelets has been shown to inhibit the growth of certain cells in culture (Tucker et al., 1984 Science 226:705-707) TGF-.beta.1 has also been shown to inhibit the growth of several human cancer cell lines (Roberts et al , 1985, Proc Natl Acad Sci. USA 82:119-123). This inhibitory/stimulatory effect of TGF-.beta.1, may depend on several factors including cell type and the physiological state of the cells (for review see Sporn et al., 1986, Science 233:532-534).
TGF-.beta.2, like TGF-.beta.1, is a polypeptide of molecular weight 26,000 composed of two identical 13,000 dalton subunits which are disulfide linked (Chiefetz et al., 1987, Cell 48:409-415; Ikeda et al., 1987, Biochemistry 26:2406-2410) and has been isolated from bovine demineralized bone (Seydin et al., 1987, J. Biol. Chem. 262:1946-1949), porcine platelets (Cheifetz et al., 1987, 48:409-415), a human prostatic adenocarcinoma cell line, PC-3 (Ikeda et al., 1987, Biochemistry 26:2406-2410), and a human gliablastoma cell line (Wrann et al., 1987, EMBO 6:1633-1636). cDNA clones coding for human and simian TGF-.beta.2 have been isolated (Madisen et al., 1988, DNA 7:1-8; Webb et al., 1988, DNA 7:493-497). The mature TGF-.beta.2 monomer is cleaved from one of two larger precursor polypeptides, the mRNAs of which may arise via differential splicing (Webb et al., 1988, DNA 7:493-497).
TGF-.beta.1and TGF-.beta.2 share 71% amino acid sequence identity in their mature regions, and 41% identity in their precursor structures. TGF-.beta.3, the amino acid sequence of which has very recently been deduced from cDNA clones, appears to contain a C-terminal 112 amino acid sequence with about 80% homology to the mature monomers of TGF-.beta.1, and TGF-.beta.2 (Dijke et al., 1988, Proc. Natl. Acad. Sci. USA 85:4715-4719). TGF-.beta.1.2 is a heterodimeric form comprising a .beta.1 and .beta.2 subunit linked by disulfide bonds (Cheifetz et al., 1987, Cell 48:409-415).